Lung Volumes

The volume of gas in the lungs can be divided into different components, and these individual volumes can be useful in diagnosing certain pulmonary diseases. Figure 6 shows the different volumes that can be measured with a spirometer. A spirometer measures the volume inspired or expired by a subject through a mouthpiece connected to a container with a water seal (as in Fig. 6) or a collapsible bellows (which is more common today). The patient wears a nose clip to ensure that an entire inhalation or exhalation is collected.

Total lung capacity (TLC) is the maximum volume that can be contained by the lungs in vivo, and it includes several different volumes. The convention is that capacities are composed of volumes that can be measured independently. Residual volume (RV) is the one volume that cannot be measured with a spirometer because it is

the volume of gas remaining in the lungs after a maximal expiratory effort. Therefore, absolute values of TLC and the functional reserve capacity (FRC) cannot be measured with a spirometer. FRC is the volume of gas left in the lungs at the end of a normal passive expiration.

Tidal volume (VT) is the normal volume inspired and expired with each breath. Inspiratory reserve volume (IRV) is the maximum volume that can be inspired above the end of a normal inspiration, and the expiratory reserve volume (ERV) is the maximum volume that can be forcibly exhaled after a normal expiration (i.e., below FRC). Inspiratory capacity (IC) is the maximum volume that can be inspired from FRC and the vital capacity (VC) is the maximum volume that can be inhaled or exhaled in vivo.

One method used to measure RV or FRC is that of gas dilution, which is another application of the principle of conservation of mass as illustrated in Fig. 7. The initial

FIGURE 8 Measurement of FRC with a body plethysmograph. Pressure (P) and volume (V) changes measured when the subject attempts to inspire against a closed airway are used to calculate lung volume as described in the text (P ■ V = k).

FIGURE 8 Measurement of FRC with a body plethysmograph. Pressure (P) and volume (V) changes measured when the subject attempts to inspire against a closed airway are used to calculate lung volume as described in the text (P ■ V = k).

lung volume (V2), which can be FRC or RV, contains none of a tracer gas such as helium. After the mouthpiece is opened to the spirometer, the individual rebreathes in and out of the spirometer until the tracer gas concentration equilibrates in the lung volume and the spirometer. O2 can be added to the spirometer to replace that consumed and CO2 can be removed by CO2 absorbents. If the final tracer gas concentration (C2) is measured, and the initial spirometer volume (Vi) and tracer gas concentration (C1) are known, then one can solve for the unknown lung volume (V2).

Another method used to measure FRC is that of a body plethysmograph as illustrated in Fig. 8. A plethys-mograph is a sealed box in which an individual can sit and breathe through a mouthpiece connected outside the box. The mouthpiece is sealed at a specified lung volume, such as FRC, and the person makes an inspiratory effort. First, Boyle's law is used to calculate the change in box volume (AV) from the known initial box volume (Vl) and pressure (P1), and the final box pressure (P2) measured during inspiratory effort:

Next, Boyle's law is applied to the lung where FRC is the unknown initial lung volume and P3 and P4 are the initial and final lung pressures measured at the mouth:

The decrease in box volume equals the increase in lung volume (AV), and the equation can be solved for FRC.

The measurement of lung volumes can be a useful diagnostic tool for pulmonary disease. For example, FRC increases with emphysema or chronic obstructive lung disease. However, the methods for measuring lung volumes can also be affected by disease. Gas dilution depends on the tracer gas reaching all parts of the lung volume, which may not occur with lung disease and gas trapping in obstructed distal airways. In contrast, the plethysmograph method will also measure trapped gas volumes within the thorax.

Normal lung volumes also provide important lessons about respiratory physiology in healthy individuals. TLC is more than 10 times larger than the normal Vt, so there is a tremendous reserve capacity for increased ventilation with increased O2 demand or reduced supply. This is part of the reason why pulmonary gas exchange is usually not a limiting factor in O2 uptake at sea level in anyone except highly trained elite athletes. It is also important that the FRC is over three times larger than normal VT. Ventilation is tidal (i.e., in and out), so oscillations in alveolar gas composition can occur during the breathing cycle. However, the large FRC relative to VT dilutes these oscillations in alveolar gas composition so PO2 and PCO2 in blood leaving the pulmonary capillaries is almost constant (±2-4 mm Hg).

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